Used to specify and override the default API & behavior in the
generated code for client languages, from what you would get from
the OpDef alone. There will be a set of ApiDefs that are common
to all client languages, and another set per client language.
The per-client-language ApiDefs will inherit values from the
common ApiDefs which it can either replace or modify.
We separate the API definition from the OpDef so we can evolve the
API while remaining backwards compatible when interpretting old
graphs. Overrides go in an "api_def.pbtxt" file with a text-format
ApiDefs message.
WARNING: Be *very* careful changing the API for any existing op --
you can change the semantics of existing code. These changes may
need to wait until a major release of TensorFlow to avoid breaking
our compatibility promises.